I might be wrong, but I believe that the entire Western civilization has been going to the left (at least from social ideology, when it comes to economics it is a very different manner). Women rights and then getting high positions in politics and economy, LGBT rights, abortion, the fight against racism, all have been progressing on the left direction (obviously with some bumps here and there). With the changes in the population of the US, most of boomers who have been very conservative dying in the next few decades, the young people tend always to be more liberal, the people who come to the US too, the average voter is gonna be more left wing than right now.
Which would mean that both parties will go to the left of what they are now. At the end, the parties have to follow their own voters, otherwise they would lose voters.